Free cutting alloys



Patented Apr. 13, 1937 UNlTD STTES ATE vlI arm FREE CUTTING ALLOYS Pennsylvania No Drawing. Application December 28, 1935,

' Serial No. 56,550

Claims.

The invention relates to aluminum base alloys and is particularly concerned with aluminum base alloys containing copper, tin and cadmium.

Aluminum base alloys containing between 3 per cent and '12 per cent of copper have been widely used heretofore. good casting characteristics and, increases the tensile strength, yield strength, and hardness of the alloy. In the lower portion of the copper range the alloy may be mechanically deformed by the Well known commercial processes such as rolling, forging, drawing, or extrusion. Alloys containing more than about 6 per cent of copper are generally used in the cast condition. Throughout the entire range of 3 to 12 per cent copper, however, the alloys are susceptible to variations and improvements in their physical properties by thermal treatments.

The addition of from 0.05 to 2 per cent of cadmium to the foregoing type of alloy accelerates to some extent the aging characteristics after the commercial scale by the processes of rolling or forging. The alloy should be extruded or cast.

In these forms, however, it is useful for many purposes where the strength obtainable in a heat treated and aged aluminum base alloy is desired.

Our invention which is described hereinbelow, is

directed to improving the machining quality of such alloys.

There are some applications wherein alumi num-copper-tin-cadmium alloys as hereinabove disclosed might be conveniently and profitably used except for an inherent disadvantage which to some extent militates against their use in the production of certain articles requiring exacting machining operations. Mechanical cutting operations such as drilling, shaping,or lathe-cutting are successfully carried out only by using certain precautions which increase the cost. of production and perhaps favor the choice of another metal or alloy which can be machined more readily but which is not so desirable in other respects, as for example, in physical properties. When alloys are diificult to machine this disadvantage becomes evident, in many .cases,

through rapid wear of the cutting tool edge, so

The copper imparts which often fouls the tool or the moving parts of the machine.

The 'aluminum-copper-tin-cadmium alloys are superior to aIuminum copper-tin alloys with respect to machining characteristics but in some applications where there is need for exceptional cutting qualities it is possible to eifect a'further improvement in the aluminum-copper-tin-cam mium alloys as will hereinafter appear.

Accordingly an object of our invention is the provision of an aluminum base alloy containing from about 3 per cent to about 12'per cent of copper, from 0.2 to 2.0 per cent of tin, and from 0.05 to about 2 per cent of cadmium which may be readily and economically machined.

Our invention resides in the-discovery that the foregoing object is effected by the addition of one or more of the elements lead, thallium or bismuth. The aluminum base alloys to which these elements are added in the proportions specified below, are known as free cutting or free machining alloys because they can be machined more rapidly than similar alloys without these elements and yet have as good or a better finished surface. After an extended series'of investigations we have discovered that thesethree metals when added to aluminum base alloys. form a class of alloying elements byreason of their favorable efiect upon the machiningi'properties of these alloys. In recognition of this effect we term' lead, thallium and bismuth free machining elements. Although the presence of one of these elements improves the machinability, we have further discovered that the simultaneous presence of two or more of these elements I is productive of an improvement in free machining characteristics which is considerably greater than that caused by the presence of the same total amount of a single free machining element. For example, the addition of 0.5 per cent lead and 0.5 per cent of bismuth to an aluminum base alloy containing about 5 per cent of copper, 0.25 per cent of tin and'0.2 per cent of cadmium effects a greater improvement in machiningquality than does the addition of 1.0 per cent of either lead or bismuth singly.

These three elements, in conjunction with the cadmium and tin in the base alloy, are unique with respect to their'eifect on the machining characteristics of aluminum-copper-tin-cadmium alloys. It is a fortlmate circumstance, therefore, that they are also of relatively low melting point, a. fact which makes possible their addition to molten aluminum in the pure state, without the intervention of so-called rich alloys. As a matter of fact, we have observed that of all the metals whosemelting point is lowcutting characteristics to an aluminum-copper,

tin-cadmium alloy, but do not have an undesirable efiect on the fundamental physical properties of this base alloy.

The total amount of the added free machining elements should not be less than about 0.05 per cent since below thisamount there is scarcely any advantageous effect. We have determined that a maximum limit of about 6v per cent total of one or more'of the free machining elements disclosed is suflicient for satisfactory commercial results, since although the free machining efiect persists beyond this amount, certain of the other physical properties may be unfavorably aflected.

Aluminum-copper-tin-cadmium alloys containing one or more of the free machining elements lead, thallium and bismuth may be machined more rapidly, with less tool wear, less tool sharpening, better quality of chip and better machined surface than the same base alloys without the free machining additions, and in fact better than the same base alloys containing an equivalent total amount of a single free machining element. i

For a preferred alloy with excellent casting characteristics we suggest one alloy containing 10 per cent of copper, 0.5 per cent tin, 0.5 per cent cadmium and a total of 3 per cent of free machining elements, the balance being aluminum.

Another alloy with excellent cutting characteristics is one containing about 6 per cent copper, 0.25 percent tin, 0.2 per cent cadmium and 0.5

per cent bismuth, the balance being aluminum.

As an example of an alloy containing added bismuth and lead we suggest one containing about 6 per cent copper, 0.25 per cent tin, 0.2 per cent cadmium, 0.2 per cent bismuth and 0.2 per cent lead, the balance being aluminum.

- For certain purposes, notably the improvement of tensile strength, hardness andgrain structure, the alloys as hereinabove disclosed may be improved by the addition of one or more of the group of elements composed of molybdenum, vanadium, titanium, tungsten, zirconium and chromium. From 0.05 to 1 per cent of any one of these elements may be used alone, but if more than one is employed the total amount should not exceed about 2 per cent.

It is characteristic of the three elements, lead,

. thallium and bismuth, and of the tin and cad- -mium in the base alloy, that they form with aluminum a series of alloys of limited liquid solubility. We have reason to believe that the free machining elements are the only elements which exhibit this characteristic, with the possible exception of several metals which are not regarded as having any commercial promise as additions to aluminum base alloys. vWithin the range disclosed and claimed however the free cutting elements may be added without unusual difllculty. We suspect that this characteristic feature of the disclosed elements may be one of 7 the significant factors which contribute to their free machining eitect. We believe that this effect is further strengthened by distributing the free machining constituent relatively homogeneously throughout the solid matrix, since these free machining constituents are practically insoluble in the solid aluminum base.

The free machining alloys which have been describedhereinabove may be subjected to the thermal treatments well known in the art to improve their strength and hardness. We have found that a solution'heat treatment and subsequent aging does not impair thefree machining quality of the alloys and in many instances the treatment even tends to improve this property.

' For many purposes a relatively high strength and hardness are necessary to the successful performance of the machined article and hence the alloy must be heat treated. This treatment is generally applied prior to the machining operation.

As hereinabove indicated the free machining elements, by reason of their low melting point, may be added to the molten aluminum alloy in pure metallic form. However, since some difllculty may be encountered in introducing them in the higher percentages of our disclosed range we prefer to use the method which is more fully described in US. Patent No. 1,959,029, issued March 15, 1934. Briefly it involves heating the melt to a somewhat higher temperature than is customary, and vigorously stirring it in excess of a critical period of time.

The term aluminum as used herein and in the appended claims embraces the usual impurities found in aluminum ingot of commercial grade or picked up in the course of the ordinary handling operations incident to melting practice.

We claim:

1. An aluminum base alloy consisting of about 6 per cent copper, 0.25 per cent tin, 0.2 per cent cadmium, and 0.5 per cent bismuth, the balance being aluminum.

2. An aluminum base alloy consisting of about 6 per cent copper, 0.25 per cent tin, 0.2 per cent cadmium, 0.2 per cent bismuth and 0.2 per cent lead, the balance being aluminum.

3. A free cutting alloy containing from 3 to 12 per cent copper, from 0.2 to 2 per cent tin and a total of from 0.05 to 6 per cent of at least two of the elements from the following metals, lead, thallium, cadmium, and bismuth, to improve its machining properties, one of said elements being inallcasescadmium,the balancebeingsubstantially aluminum. v

4. A free cutting alloy containing from 3 tell! percent copper, from 0.2 to 2 per cent tin,.'a total of from 0.05 to 6 per cent of at least two of the elements from the following metals, lead, thallium, cadmium, and bismuth, to improve its machining properties, one of said elements being in all cases cadmium, and from 0.05 to 2-per cent of hardening metal from the group composed of molybdenum, vanadium, titanium, tungsten, zirconium, and chromium, the balance being substantially aluminum.

5. An aluminum base alloy consisting of about 6 per cent copper, 0.25 per cent tin, 0.2 per cent cadmium, and 0.2 per cent lead, the balance being WALTER A. DEAN. 

